Patent literature 1 discloses a background art in this technical field. According to this literature, in Multi Scale Retinex Processing, a composite blur image is generated by selecting, for each pixel, any of a plurality of blur images with different degrees of blurring, which are generated by a plurality of peripheral functions with different scales, in accordance with the pixel value levels of an original image to be processed. The literature describes that a low-pass filter is applied to the composite blur image to prevent the unnatural discontinuity of boundaries and perform Retinex Processing (see the abstract).